The fact that we can use metamaterials to realise a wide range of material parameters means there is actually more to explore in these classical systems (the distribution of atoms in an ordinary crystal is not easy to specify on the scale of an electron wavelength!). Work on topological photonic and acoustic materials is therefore able to investigate effects that could not be observed in condensed matter systems (see the discussion in e.g. [1]} and [2]}), and even allows the exploration of active topological non–Hermitian materials, where the material can amplify an absorb the wave in a controlled way [3]} (which is also extremely difficult to mimic in electronic systems).
